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Total Body Irradiation

Definitions

The following definition is partly based on StrålevernRapport 2003:13, which is referring to ICRU50 and 62 as well as NACP.

 

Definitions of target volumes in accordance with the ICRU (International Commission on Radiation Units and Measurements)

 

GTV (Gross tumor volume)

Gross palpable or visible/identifiable area of malignant growth.

 

CTV (Clinical target volume)

Macroscopic tumor volume including any remaining tumor tissue.

 

ITV (Internal Target Volume)

Volume containing CTV and internal margin to allow for internal movements and changes to CTV.

 

PTV (Planning Target Volume) Geometric volume containing ITV with set-up margin taking into accound patient movements, variations in patient positioning, and field settings.
OAR (Organ-at-Risk) Normal tissue senstive to radiation that may significantly affect planning and/or dose.

PRV (Planning organ-at-risk volume)

 

Geometric volume containing risk volume with set-up margin.
TV (Treated Volume) Volume within an isodose surface considered sufficient based on the treatment intention.
IV (Irradiated Volume) Volume-to-receive dose that is of significance with regard to normal tissue tolerance.
CI (Conformity Index) Relationship between the planning target volume and treated volume (PTV/TV).

 

Field Limits

The field limit is defined as the required course for the 50% isodose curve outside the target volume to give a therapeutic isodose (90% isodose) to the target volume which is intended to be treated. The distance from 90-50% of the isodose (penumbra) depends on multiple conditions and is typically 5-7 mm.

Definition of margins

For radiation therapy of malignant lymphomas, a table is formulated which summarizes standards used for GTV, margins for CTV and ITV, as well as shaping of field limits.

 

Target volume for radiation therapy
GTV Current tumor for indolent NHL stage I/II1, original tumor(before chemotherapy minus balloon effect) for aggressive NHL stage I/II1 and HL stage I/IIA

Residual tumor for aggressive NHL stage II2/IV and HL stage IIB/IV

 

CTV GTV + 2 cm craniocaudal for limited disease/short chemotherapy

GTV + 1 cm craniocaudal for residual tumor from extensive disease after full chemotherapy

GTV + 1cm in transverse plane

CTV should always contain the entire lymph node region in the levels to be radiated (limited for lungs and bone, if there is no suspicion of infiltration).

CTV may for indolent NHL stage I/II1 contain the nearest unaffected lymph node region or parts of it.

 

ITV CTV if internal movement can be ignored (CNS, ENT)

CTV + 1 cm craniocaudal and + 0.5 cm transverse in the mediastinum

CTV + 2–3 cm in mesentery

CTV + 0-0.5 cm transverse retroperitoneally

 

PTV

Not routinely defined

 

Field limits ITV + Setup margin and penumbra (1.2 cm)

The field limits should be such that later junctions are simple (on one side of the spine, in vertebral discs etc.)

 

Involved node

The radiation field which surrounds the macroscopically involved lymph node only with margin. Thus far, this definition is rarely used in Norway, but increasingly in international studies.

 

Involved field

Radiation field which includes the involved macroscopic lymph node region or organ with margin. After limited chemotherapy for localized lymphomas, the originally affected macroscopic area is used as a basis for field shaping (with the exception of the balloon effect). For residual changes after full chemotherapy in advanced stages, the residual tumor is usually used as a basis (multiple exceptions). What are adequate margins from the macroscopic tumor to the field limit depend on multiple factors. For early stages of NHL and HL without previous chemotherapy or after chemotherapy (3-6 CHOP-based treatments, 2-4 ABVD or equivalent), the margins from the initial extension to the field limit should be 3-4 cm in the vertical direction, from the initial extent and 2 cm in the transversal plane (with the exception of the balloon effect). For residual changes after full chemotherapy for advanced NHL and HL and relatively little internal mobility, then 2 cm from the residual tumor to the field limit is used. Wider margins must be considered in areas of large internal mobility (abdomen, structures near the diaphragm). Regularly, for nodal involvement, the target volume includes the entire lymph node region in the transversal plane for those levels included in the field.

Traditionally, the entire involved lymph node region has been included completely in the craniocaudal direction (direction for lymph drainage). This provides a recognizable geometric field (parts of mantle field or inverted Y-field) which has advantages for standardizing, reproducibility, later junctioning etc. The lymph node regions, as they are defined in the Ann-Arbor classification, represent no functional biological unit and are not intended as a basis for radiation therapy. In this way, it is natural to see the regions as coherent in the vertical direction of the lymph drainage and to use margins to the involved lymph nodes to avoid radiation of entire regions (for example neck/supraclavicular region, mediastinum, and retroperitoneum). Parts of the neighboring regions may be included to compensate for the minimum margins given above. Field shaping should still follow the geometric forms as much as possible, making later field junctioning easier and to avoid border recurrences in areas which are difficult to re-irradiate.

For extranodal lymphomas/organ manifestations, the entire organ is sometimes included (thyroid gland, stomach, brain, spinal cord). Internal mobility must also be taken into consideration here, for example stomach movement, movement of lungs etc. For several organ localizations, it is not possible to give full doses to the entire organ due to the tolerance for ionizing radiation (lungs, liver, kidney), and the fields/doses must be adapted accordingly.

Extended field

This concept is utilized for fields which include macroscopically involved regions/organs and lymph node regions where it is assumed there is microscopic disease. This may be the nearest macroscopic normal region or multiple, more distant areas. The concept was developed for Hodgkin's lymphoma at a time when radiation therapy was the only modality used and was given to large areas with assumed microscopic disease on one or both sides of the diaphragm (mantle field, paraaortal field, inverted Y-field). For today's purposes, the concept is not of much benefit. For localized stages of low-grade NHL, where radiation therapy is given alone with the intention of curing the disease, we have chosen to include the nearest unaffected region in the radiation field, that is, a "minimally extended field." However, this is not practiced at all radiation therapy centers.

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